Types of bacteriological culture media

Culture media are classified in several ways:

1. Based on physical state

• Solid media — contain 1.5–2.0% agar (e.g., nutrient agar, blood agar). Used for isolation of colonies and pure culture.
• Semi-solid media — contain 0.2–0.5% agar (e.g., motility test medium, SIM). Used mainly for motility and anaerobic tests.
• Liquid (broth) media — no agar (e.g., nutrient broth, peptone water). Used for enrichment and for MPN tests.

2. Based on chemical composition

• Synthetic (defined) media — exact chemical composition known (e.g., Czapek–Dox agar, minimal salt medium).
• Non-synthetic (complex/undefined) media — composition not exactly known, made from peptone, beef extract, yeast extract (e.g., nutrient agar, MacConkey agar).

3. Based on purpose/function

• General purpose (basal) media — support growth of most non-fastidious organisms (nutrient agar, nutrient broth).
• Enriched media — basal medium plus blood, serum, egg, etc. (blood agar, chocolate agar) for fastidious organisms.
• Enrichment media — liquid media that suppress unwanted flora and increase the number of the desired organism (selenite F broth, alkaline peptone water).
• Selective media — contain inhibitors (dyes, bile salts, antibiotics) that allow only certain organisms to grow (MacConkey agar, Mannitol salt agar, Wilson–Blair medium).
• Differential media — allow colonies of different organisms to be distinguished by colour change (MacConkey agar, blood agar).
• Transport media — for safe transport of specimens without multiplication (Stuart's, Amies, Cary–Blair).
• Anaerobic media — for anaerobes (Robertson's cooked meat medium, thioglycollate broth).
• Assay media — for vitamin, amino acid and antibiotic assays.

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Enumeration by Most Probable Number (MPN) method

Principle The MPN method is a statistical, multiple-tube dilution method used to estimate the number of viable organisms (especially coliforms) in a sample when the count is too low to be detected by plate count. It is based on the probability that the sample contains at least one viable cell in the diluted portions tested. Results are read from probability tables (McCrady's tables) and expressed as MPN per 100 mL of sample.

Materials

• Sample (e.g., water)
• Lactose broth (single and double strength) with Durham's tubes
• Brilliant green lactose bile broth (BGLB) or MacConkey broth
• Eosin methylene blue (EMB) agar / Endo agar
• Inoculating loop, sterile pipettes, incubator

Procedure

1. Presumptive test

• Set up three series of fermentation tubes: 5 tubes of double-strength lactose broth (10 mL each) and 10 tubes of single-strength lactose broth (10 mL each).
• Inoculate 10 mL of sample into each double-strength tube, 1 mL into each of 5 single-strength tubes, and 0.1 mL into each of the remaining 5 single-strength tubes.
• Incubate at 35–37 °C for 24 h; if no gas, re-incubate up to 48 h.
• Tubes showing gas in the Durham tube are recorded as positive.
• The number of positive tubes in the three sets (10 mL, 1 mL, 0.1 mL) is noted as a three-digit code (e.g., 5-3-1) and the MPN/100 mL is read from the MPN table.

2. Confirmed test

• Transfer a loopful from each positive presumptive tube into a tube of BGLB broth.
• Incubate at 35–37 °C for 24–48 h.
• Gas production confirms the presence of coliforms. The confirmed MPN is read from the table using the new positive-tube code.

3. Completed test

• Streak from a positive BGLB tube onto EMB or Endo agar; incubate at 37 °C for 24 h.
• Typical coliform colonies (green metallic sheen on EMB) are picked and transferred to lactose broth and nutrient agar slants.
• Gas in lactose broth plus Gram-negative, non-spore-forming rods on the slant completes the test and confirms E. coli.

Interpretation The MPN value is read from the standard MPN table for the combination of positive tubes. For example, a code of 5-3-1 corresponds to an MPN of 110 per 100 mL. The 95% confidence limits are also given in the table, showing the statistical nature of the estimate.

Advantages

• Simple, inexpensive, no special equipment.
• Useful for low-count samples (drinking water, milk).
• Detects viable cells directly.

Limitations

• Only a statistical estimate, not an exact count.
• Laborious and time-consuming (up to 48–72 h).
• Less precise than the plate count method.
• Cannot distinguish between closely related organisms without confirmatory tests.

Structure of the cell wall of Gram-positive bacteria

The cell wall of Gram-positive bacteria is thick, rigid and relatively simple in chemical composition. It lies outside the cytoplasmic membrane and is 20–80 nm thick. Its main structural features are:

1. Peptidoglycan (murein) — the dominant component. It forms 60–90% of the wall's dry weight. It is a mesh-like polymer made of glycan strands of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues, cross-linked by short peptide chains (tetrapeptides) through peptide bridges. In Staphylococcus aureus, the bridge is a pentaglycine chain. The high degree of cross-linking gives the wall great mechanical strength and rigidity. The tetrapeptide attached to NAM is usually L-alanine–D-glutamic acid–L-lysine–D-alanine (in most Gram-positives; diaminopimelic acid in some).

2. Teichoic acids. These are acidic, water-soluble polymers of glycerol or ribitol phosphate (or sugar alcohols) that are covalently linked either to the peptidoglycan (wall teichoic acid) or to the cytoplasmic membrane (lipoteichoic acid). They give the wall a negative charge, help in binding divalent cations (Mg²⁺), contribute to antigenicity and virulence, and regulate autolysin activity. They are absent in Gram-negative bacteria.

3. Absence of an outer membrane. There is no outer membrane, no periplasmic space of the Gram-negative type and no lipopolysaccharide (LPS). The wall is therefore directly exposed to the environment.

4. Other components. Some species contain surface proteins (e.g. protein A in S. aureus, M protein in Streptococcus pyogenes), polysaccharides (e.g. teichuronic acid) and, in acid-fast Gram-positives such as Mycobacterium, a waxy mycolic acid layer.

Because of this thick, highly cross-linked peptidoglycan layer, the wall retains the crystal violet–iodine complex during Gram staining and the cells appear purple/violet even after alcohol decolourisation.

Differences between Gram-positive and Gram-negative bacteria

Thus the Gram-positive wall is a thick, teichoic-acid-containing peptidoglycan layer without an outer membrane, and this difference in wall architecture is the basis of the Gram staining reaction and of the differing susceptibility of the two groups to antibiotics and lysozyme.

Passive and Active Transport Systems in Bacteria

Bacteria are surrounded by a plasma membrane (cytoplasmic membrane) that is selectively permeable. Since bacteria lack membrane-bound organelles, all transport of nutrients, ions, and waste products across the membrane occurs through transport systems located in this membrane. These systems are broadly classified into passive transport (does not require metabolic energy) and active transport (requires energy, usually ATP or a proton motive force).

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1. Passive Transport

In passive transport, substances move down their concentration gradient (from higher to lower concentration) without the expenditure of cellular energy. The driving force is the electrochemical gradient. It includes:

a) Simple Diffusion

• Small, non-polar molecules such as O₂, CO₂, H₂O, glycerol, and some small hydrophobic molecules pass directly through the lipid bilayer.
• The rate depends on the concentration gradient and the solubility of the molecule in the lipid bilayer.
• No carrier protein is involved; it is a non-specific process.

b) Facilitated Diffusion

• Polar or charged molecules (e.g., glycerol, some sugars, amino acids, ions) cannot pass through the hydrophobic lipid bilayer easily. They require specific carrier (transport) proteins.
• The carrier protein undergoes a conformational change to allow the solute to cross the membrane.
• It is saturable (carrier proteins have a limited number of binding sites), specific, and shows competitive inhibition.
• Examples: Glycerol facilitator (GlpF) in E. coli, lactose permease (in some contexts), and various ion channels.

c) Osmosis

• Movement of water across a semi-permeable membrane from a region of higher water potential to lower water potential.
• In bacteria, water moves through the membrane via aquaporins (water channels) or by simple diffusion.

Key features of passive transport:

• No energy (ATP) required.
• Movement is down the concentration gradient.
• Carrier proteins are involved only in facilitated diffusion.
• It is a reversible process.

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2. Active Transport

In active transport, substances move against their concentration gradient (from lower to higher concentration). This requires metabolic energy, usually in the form of ATP or the proton motive force (PMF). Active transport systems are of two main types:

a) Primary Active Transport

• Directly uses ATP as the energy source.
• The transport protein itself has ATPase activity.
• Example: ABC transporters (ATP-Binding Cassette transporters).
  • They consist of:
    • A substrate-binding protein (in Gram-negative bacteria, located in the periplasm).
    • Membrane-spanning proteins (permeases).
    • ATP-binding cassette (on the cytoplasmic side).
  • Mechanism: The substrate binds to the binding protein → it is delivered to the membrane permease → ATP is hydrolyzed → conformational change → substrate is released into the cytoplasm.
  • Examples: Histidine permease, maltose transport system, phosphate transport system in E. coli.

b) Secondary Active Transport

• Does not directly use ATP. Instead, it uses the proton motive force (PMF) or ion gradients established by primary transport or electron transport chain.
• Two types:
  1. Symport (Co-transport): The solute and the driving ion (usually H⁺) move in the same direction.
     • Example: Lactose permease (LacY) in E. coli — H⁺ and lactose enter together.
  2. Antiport (Counter-transport): The solute and the driving ion move in opposite directions.
     • Example: Na⁺/H⁺ antiporter — Na⁺ is expelled while H⁺ enters.

c) Group Translocation

• A special type of active transport in bacteria where the substrate is chemically modified during transport.
• Example: Phosphotransferase System (PTS) for glucose uptake in E. coli.
  • Glucose is phosphorylated to glucose-6-phosphate as it enters the cell.
  • Energy is derived from phosphoenolpyruvate (PEP).
  • The substrate is trapped inside the cell because the phosphorylated form cannot pass back through the membrane.

Key features of active transport:

• Requires energy (ATP or PMF).
• Movement is against the concentration gradient.
• Carrier proteins are specific and saturable.
• It is an irreversible, unidirectional process.
• It allows bacteria to accumulate nutrients even when their external concentration is very low.

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Comparison Table: Passive vs Active Transport

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Significance in Bacteria

1. Passive transport allows rapid uptake of small molecules and waste removal without energy cost.
2. Active transport enables bacteria to:
   • Scavenge nutrients from very dilute environments.
   • Maintain internal ion concentrations (e.g., K⁺, Na⁺) different from the external medium.
   • Accumulate sugars, amino acids, and other metabolites against a concentration gradient.
   • Expel toxic substances and antibiotics (efflux pumps).
3. The proton motive force generated by active transport also drives flagellar rotation and ATP synthesis.

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Conclusion

Bacteria use a combination of passive and active transport systems to regulate the internal environment and obtain nutrients. Passive transport is simple and energy-free but limited to movement down gradients. Active transport is energy-dependent but allows bacteria to survive in nutrient-poor and hostile environments by accumulating essential molecules and expelling harmful ones. Together, these systems are crucial for bacterial growth, metabolism, and survival.

Methods for the Cultivation of Viruses

Viruses are obligate intracellular parasites — they lack their own metabolic machinery and can multiply only inside living host cells. Therefore they cannot be grown on ordinary artificial (bacteriological) media; they require living cells, tissues, or embryos. The principal methods for cultivating viruses are:

1. In Vivo (Animal) Inoculation

• A susceptible living animal (e.g., mice, guinea pigs, rabbits, monkeys) is inoculated with the virus by a suitable route (intracerebral, intranasal, intraperitoneal, etc.).
• The animal is observed for signs of disease; the virus is then harvested from its tissues/organs/body fluids.
• Examples: Mice for influenza, rabies, and yellow fever viruses; monkeys for poliovirus (historically).
• Uses: Isolation of unknown viruses, study of viral pathogenesis and oncogenesis, production of vaccines and antisera, and testing of antiviral drugs.
• Limitations: Costly, time-consuming, ethical concerns, and animals may harbour their own latent/contaminating viruses.

2. Embryonated Egg (Chick Embryo) Technique

• Fertilized hen's eggs incubated for 8–12 days are used, since the embryo is then well developed but still susceptible.
• The virus is inoculated into a specific site depending on the virus:

• Advantages: Sterile, cheap, easy to maintain, and yields large quantities of virus.
• Uses: Primary isolation of viruses and large-scale vaccine production (e.g., influenza vaccine).
• Limitations: Not all viruses grow in eggs; eggs may contain contaminating viruses/antibodies.

3. Tissue (Cell) Culture — the most widely used method

This is the preferred modern method. It is of three types:

(a) Organ Culture

• Small fragments of living tissue (e.g., trachea, intestine) are maintained in vitro with their natural architecture preserved.
• The virus grows in the natural tissue.
• Limitation: Difficult to maintain for long periods.

(b) Explant Culture

• Tissue fragments are placed in a medium; cells grow out from the explant.
• Used for growing viruses such as adenoviruses.

(c) Cell Culture (Cell Line Culture) — most important

• Primary cell culture: Cells taken directly from host tissue (e.g., monkey kidney cells) and grown once; susceptible to many viruses but cannot be subcultured indefinitely.
• Diploid (semi-continuous) cell line: Cells subcultured 50–100 times while retaining the diploid chromosome number (e.g., human embryonic lung fibroblasts — WI-38); used for vaccine production.
• Continuous (heteroploid) cell line: Cells subcultured indefinitely (e.g., HeLa, Hep-2, Vero cells); convenient but may differ from normal cells.

Detection of viral growth in cell culture:

• Cytopathic effect (CPE) — cell rounding, fusion, or death.
• Hemadsorption — attachment of red blood cells to infected cells.
• Plaque formation — clear zones in a cell monolayer.
• Metabolic inhibition / interference tests.

4. Comparison of the Main Methods

Conclusion

Of all the methods, cell culture is the most important and widely used today because it is sensitive, economical, and can be standardized. Embryonated eggs remain valuable for vaccine production, while animal inoculation is reserved for special studies. Together, these methods allow the isolation, identification, and large-scale production of viruses for diagnosis and vaccine development.

Sterilization is the process of complete destruction or removal of all forms of microbial life, including bacteria, viruses, fungi, spores, and prions, from an object or medium. A sterilized item is absolutely free of viable microorganisms.

Filtration Sterilization

Principle

Filtration sterilization works on the principle of physical removal of microorganisms rather than killing them. The liquid or gas is passed through a filter having pores smaller than the microorganisms. Since bacteria (typically 0.5–5 µm) and spores are larger than the pore size of the filter (usually 0.22 µm), they are trapped on the filter surface or within its matrix, and the filtrate that passes through is sterile. It is a cold sterilization method, so it does not involve heat and is therefore suitable for heat-labile substances.

Procedure

1. Assembly: The filtration apparatus (e.g., membrane filter holder, syringe filter, or candle filter) is properly assembled under aseptic conditions. The filter membrane (commonly cellulose nitrate, cellulose acetate, or PTFE with 0.22 µm pore size) is placed in the holder.
2. Pre-sterilization: The apparatus and membrane are pre-sterilized by autoclaving or ethylene oxide before use.
3. Filtration: The liquid to be sterilized is forced through the membrane either by:
   • Positive pressure (using a syringe or pressure pump), or
   • Negative pressure/vacuum (using a vacuum flask and pump). The microorganisms are retained on the membrane while the sterile filtrate is collected in a sterile container.
4. Aseptic collection: The filtrate is collected aseptically into a pre-sterilized receiver.
5. Post-filtration: The membrane may be removed and incubated on culture media to check sterility (integrity test), and the filtrate is stored properly.

Applications

1. Heat-labile solutions: Sterilization of sera, vaccines, antibiotics, vitamins, enzymes, and hormone solutions that would be destroyed by heat.
2. Bacteriological media: Sterilization of sugar solutions and other media components that cannot be autoclaved.
3. Pharmaceuticals: Sterilization of injectable drugs, ophthalmic solutions, and biological products.
4. Air sterilization: HEPA filters used in laminar flow cabinets, operation theatres, and clean rooms to provide sterile air.
5. Water purification: Membrane filters used in water treatment and in microbiology for testing water quality.
6. Laboratory use: Sterilization of tissue culture media and reagents in cell culture work.

Bacterial Recombination

Bacterial recombination is the process by which genetic material from one bacterium (the donor) is transferred to another bacterium (the recipient) and becomes integrated into the recipient's genome, producing a recombinant cell with a new combination of genes. Unlike sexual reproduction in eukaryotes, it is not associated with reproduction or gamete formation; it is a means of genetic variation and evolution in prokaryotes. It occurs by three main mechanisms: transformation (uptake of naked DNA from the environment), conjugation (transfer of DNA through a sex pilus), and transduction (transfer of DNA mediated by a bacteriophage).

Transduction

Transduction is the transfer of bacterial DNA from one bacterium to another by a bacteriophage (bacterial virus). It was discovered by Zinder and Lederberg in Salmonella typhimurium in 1952. It is of two types: generalized and specialized.

Generalized transduction (lytic cycle):

1. A virulent bacteriophage attaches to the donor bacterium and injects its DNA.
2. The phage DNA replicates and directs the synthesis of phage proteins; simultaneously, the host DNA is degraded by phage enzymes.
3. During assembly of new phage particles, a fragment of the degraded donor bacterial DNA is mistakenly packaged into a phage head instead of phage DNA. This faulty phage is called a transducing phage.
4. The transducing phage attaches to a recipient bacterium and injects the donor DNA fragment into it.
5. Inside the recipient, the donor DNA fragment may recombine with the homologous region of the recipient chromosome (with the help of RecA enzyme), replacing the recipient's allele. The recipient cell that receives the donor gene is called a transductant and is now genetically recombinant. If recombination does not occur, the fragment is degraded.
6. Since any bacterial gene can be packaged, this is called generalized transduction (e.g., phage P1 in E. coli, phage P22 in Salmonella).

Specialized transduction (lysogenic cycle):

1. A temperate phage (e.g., phage lambda) integrates into the bacterial chromosome as a prophage at a specific attachment site.
2. On induction, the prophage is excised. Occasionally, excision is imprecise, so a small adjacent piece of bacterial DNA is carried along with the phage DNA, while some phage DNA is left behind.
3. The resulting phage particle carries both phage genes and specific bacterial genes (e.g., gal or bio genes in lambda).
4. This phage infects a recipient bacterium and injects the hybrid DNA, which integrates into the recipient chromosome.
5. Thus only genes adjacent to the prophage insertion site are transferred — hence it is called specialized (restricted) transduction.

Significance (2 marks)

1. It is a major source of genetic variation and recombination in bacteria, leading to new gene combinations and evolution.
2. It helps in gene mapping — the frequency of co-transduction indicates the distance between genes.
3. It explains the transfer of virulence and antibiotic-resistance genes between bacteria (e.g., toxin genes in Corynebacterium diphtheriae, Vibrio cholerae).
4. It is an important tool in genetic engineering and biotechnology for transferring genes into bacteria.

Basis of Classification of Bacteria

Bacteria are classified on the following bases:

1. Morphological classification (shape)

• Cocci — spherical; arranged as diplococci (pairs), streptococci (chains), staphylococci (irregular clusters), tetracocci (groups of four) and sarcinae (cubical packets of eight).
• Bacilli — rod-shaped; may be single, diplo-, strepto- or palisade in arrangement.
• Spirilla — rigid spiral forms.
• Spirochaetes — flexible, coiled forms.
• Vibrios — comma-shaped, curved rods.
• Filamentous — branched or unbranched filaments (e.g. Actinomycetes).

2. Staining behaviour

• Gram-positive — retain crystal violet (thick peptidoglycan layer).
• Gram-negative — decolourised and counterstained pink (thin peptidoglycan, outer membrane).
• Acid-fast — retain carbol fuchsin even after acid-alcohol (e.g. Mycobacterium).

3. Oxygen requirement

• Aerobes — require oxygen; obligate aerobes cannot grow without it.
• Anaerobes — grow only in absence of oxygen; facultative anaerobes grow either way.
• Microaerophiles — need low oxygen tension.
• Capnophiles — require increased CO₂.

4. Temperature requirement

• Psychrophiles (below 20 °C), Mesophiles (20–45 °C, most pathogens), Thermophiles (45–80 °C), Hyperthermophiles (above 80 °C).

5. Nutritional requirement

• Autotrophs — synthesize their own food; photoautotrophs (use light) and chemoautotrophs (use inorganic compounds).
• Heterotrophs — depend on organic matter; saprophytes, parasites and symbionts.

6. Flagellar arrangement

• Atrichous (no flagella), Monotrichous (single polar), Lophotrichous (tuft at one end), Amphitrichous (tufts at both ends), Peritrichous (all over the body).

7. Spore formation

• Sporing (e.g. Bacillus, Clostridium) and non-sporing bacteria.

8. Genetic and molecular basis

• DNA base composition — guanine + cytosine (GC) content varies from about 25 % to 80 %.
• DNA homology and rRNA sequencing — used in modern taxonomy to establish phylogenetic relationships.
• Presence of plasmids, phages and transposons also serves as a basis.

9. Serological classification

• Based on antigenic differences in cell wall (O), flagellar (H), capsular (K) and fimbrial antigens; used in typing Salmonella, Streptococcus, etc.

10. Pathogenicity

• Pathogenic and non-pathogenic bacteria.

11. Bergey's classification

• The most widely accepted system, based on morphology, staining, nutrition, oxygen relation and molecular characters, divides bacteria into about 30 groups (e.g. Gracilicutes, Firmicutes, Tenericutes, Mendosicutes).

Thus bacteria are classified on morphological, physiological, biochemical, serological and molecular bases, with molecular methods now forming the backbone of modern bacterial taxonomy.

Applications of microorganisms in food and pharmaceutical industries

A. In the food industry

1. Fermented foods and beverages — Saccharomyces cerevisiae ferments sugars to produce bread, beer, wine and jand; lactic acid bacteria (Lactobacillus, Streptococcus) produce curd, cheese, butter, gundruk, sinki and kimchi.
2. Dairy products — Streptococcus lactis and Lactobacillus bulgaricus curdle milk; Penicillium roqueforti and P. camemberti give flavour and texture to blue and Camembert cheese.
3. Food additives and supplements — Aspergillus niger produces citric acid; Acetobacter produces vinegar; Saccharomyces yields vitamin B-complex and yeast extract (Marmite).
4. Single-cell protein (SCP) — Spirulina, Chlorella and Candida utilis are grown as protein-rich food/feed supplements.
5. Enzymes — Bacillus species give proteases and amylases; Aspergillus gives pectinase (fruit juice clarification) and lactase (milk for lactose-intolerant people).
6. Food preservation and safety — fermentation produces organic acids and alcohol that inhibit spoilage organisms; bacteriocins such as nisin act as natural preservatives.
7. Food spoilage and food-borne disease — Clostridium botulinum, Salmonella, Staphylococcus aureus and Aspergillus cause spoilage and food poisoning, so their control is equally important.

B. In the pharmaceutical industry

1. Antibiotic production — Penicillium notatum/chrysogenum → penicillin; Streptomyces → streptomycin, tetracycline, erythromycin, chloramphenicol.
2. Vaccine production — attenuated or killed pathogens and their toxins are used to make BCG, DPT, polio, hepatitis B and typhoid vaccines.
3. Vitamins and enzymes — Ashbya gossypii → riboflavin (B₂); Pseudomonas denitrificans → vitamin B₁₂; microbial proteases, lipases and streptokinase are used as medicines.
4. Hormones and therapeutic proteins — recombinant E. coli and yeast produce human insulin, human growth hormone, interferon and erythropoietin.
5. Steroids and organic acids — Rhizopus and Aspergillus carry out biotransformation of steroids (cortisone, sex hormones); A. niger supplies citric acid for syrups.
6. Antibiotics and enzyme inhibitors — statins from Penicillium citrinum lower blood cholesterol.
7. Genetic engineering and gene therapy — bacteria act as hosts and vectors for cloning genes and producing gene probes and diagnostic reagents.
8. Diagnostics and quality testing — killed Salmonella typhi and S. paratyphi O and H antigen suspensions are used in the Widal test; microbial cultures and antigens are used in ELISA and sterility testing; microbial assays (e.g. Bacillus subtilis) test antibiotic potency.

Conclusion: Microorganisms are thus indispensable in both industries — as producers of fermented foods, enzymes, vitamins and antibiotics, and as tools for vaccines, recombinant drugs and diagnostics — while also being agents of spoilage and disease that must be controlled.

Characteristics of DNA

1. Double-stranded helix — DNA consists of two antiparallel polynucleotide chains wound around a common axis to form a right-handed double helix (B-DNA), with about 10 base pairs per turn and a pitch of 3.4 nm.

2. Sugar–phosphate backbone — Each strand has a backbone of deoxyribose sugar and phosphate groups joined by 3′→5′ phosphodiester bonds; the two strands run in opposite directions (antiparallel).

3. Complementary base pairing — The bases project inward and pair specifically: adenine with thymine (2 hydrogen bonds) and guanine with cytosine (3 hydrogen bonds). This makes the two strands complementary, so A = T and G = C (Chargaff's rule).

4. Constant diameter — A purine always pairs with a pyrimidine, so the helix has a uniform width of about 2 nm.

5. Carries genetic information — The sequence of bases along the strand stores hereditary information in the form of codons, which determine the amino acid sequence of proteins.

6. Self-replicating (semi-conservative) — DNA can make exact copies of itself; each daughter molecule retains one parental strand and one newly synthesised strand.

7. Mutable — The base sequence can undergo changes (mutations), which are the source of genetic variation.

8. Stable yet reactive — The covalent sugar–phosphate backbone gives DNA chemical stability, while the hydrogen-bonded base pairs allow the strands to separate easily during replication and transcription.

9. Denaturation and renaturation — On heating above about 90 °C the hydrogen bonds break and the strands separate (denaturation); on slow cooling they re-form the helix (renaturation).

10. Genetic recombination — DNA can exchange segments between homologous molecules, producing new gene combinations.

The TCA Cycle (Tricarboxylic Acid Cycle / Krebs Cycle / Citric Acid Cycle)

The TCA cycle is the final common oxidative pathway in which acetyl-CoA is completely oxidized to CO₂, with the reducing equivalents NADH and FADH₂ being generated for ATP production via oxidative phosphorylation. It occurs in the mitochondrial matrix in eukaryotes. One turn of the cycle yields 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂.

Steps of the cycle:

1. Formation of Citrate: Acetyl-CoA (2C) condenses with oxaloacetate (4C) to form citrate (6C). Catalyzed by citrate synthase. This is the rate-limiting step.

2. Isomerization to Isocitrate: Citrate is converted to isocitrate via cis-aconitate by aconitase.

3. Oxidative Decarboxylation (1st): Isocitrate → α-ketoglutarate (5C) + CO₂ + NADH. Catalyzed by isocitrate dehydrogenase (rate-limiting, allosterically activated by ADP, inhibited by ATP/NADH).

4. Oxidative Decarboxylation (2nd): α-Ketoglutarate → succinyl-CoA (4C) + CO₂ + NADH. Catalyzed by α-ketoglutarate dehydrogenase (requires TPP, lipoic acid, CoA, FAD, NAD⁺).

5. Conversion to Succinate: Succinyl-CoA → succinate + GTP. Catalyzed by succinyl-CoA synthetase (substrate-level phosphorylation).

6. Oxidation to Fumarate: Succinate → fumarate + FADH₂. Catalyzed by succinate dehydrogenase (Complex II of ETC).

7. Hydration: Fumarate + H₂O → malate. Catalyzed by fumarase.

8. Oxidation to Oxaloacetate: Malate → oxaloacetate + NADH. Catalyzed by malate dehydrogenase. Oxaloacetate regenerates to start the next cycle.

Summary equation: $\text{Acetyl-CoA} + 3\,\text{NAD}^+ + \text{FAD} + \text{GDP} + \text{P}_i + 2\,\text{H}_2\text{O} \rightarrow 2\,\text{CO}_2 + 3\,\text{NADH} + \text{FADH}_2 + \text{GTP} + \text{CoA}$

Significance: It is the common catabolic pathway for carbohydrates, fats, and proteins; provides precursors for biosynthesis (e.g., oxaloacetate for gluconeogenesis, α-ketoglutarate for amino acids); and generates the bulk of reducing power for oxidative phosphorylation.

Aseptic techniques are the set of practices and procedures used to prevent contamination of cultures, sterile media, equipment, and the worker by unwanted microorganisms, and to protect the worker and environment from the organisms being handled. In a microbiology laboratory, every operation involving pure cultures or sterile materials must be carried out aseptically.

Major aseptic techniques:

1. Sterilization of media, glassware and instruments — Media and glassware are sterilized by autoclaving at 121°C, 15 psi for 15–20 minutes. Heat-labile solutions are sterilized by membrane filtration (0.22 µm), and inoculation loops/needles are sterilized by flaming until red hot.

2. Working near a flame (aseptic zone) — Inoculations are performed in the zone of rising hot air near a Bunsen burner flame. The upward air current prevents airborne microbes from settling into open tubes or plates.

3. Flaming the mouth of containers — The neck/mouth of test tubes, bottles and flasks is passed through the flame before and after transfer to kill organisms on the rim and set up convection currents that keep contaminants out.

4. Holding vessels correctly — Tubes and plates are held at an angle, never vertically open, and lids of petri plates are lifted only slightly (clamshell style) to minimize exposure to air.

5. Transfer with sterile tools — Only sterile loops, needles, pipettes or swabs are used, and each is flamed or replaced before reuse. Pipettes are never touched at the delivery end.

6. Surface disinfection — The workbench is wiped with 70% ethanol or a suitable disinfectant before and after work; spills are flooded with disinfectant and left for the required contact time.

7. Personal hygiene and protective wear — Hands are washed with soap and disinfected; a laboratory coat, gloves and sometimes a mask and goggles are worn. Talking, coughing and unnecessary movement are avoided.

8. Laminar air flow / biosafety cabinet — For sensitive work (e.g., tissue culture, pathogenic organisms), a laminar flow hood or biosafety cabinet provides HEPA-filtered sterile air.

9. Proper handling and disposal — Plates are not opened unnecessarily; used cultures, loops and waste are discarded into disinfectant or biohazard bins and autoclaved before disposal. Cultures are never pipetted by mouth.

Importance: These techniques prevent contamination of pure cultures (which would give false results), keep media and reagents sterile, protect the worker from infection, and prevent release of organisms into the environment.

Types of Staining Methods for Bacteria

Staining is a technique used to colour microorganisms so they become visible and distinguishable under the microscope. Since bacteria are almost transparent, stains are essential for observing their shape, size, arrangement, and cellular structures. The main staining methods are classified as follows:

1. Simple Staining

A single dye (e.g., methylene blue, crystal violet, carbol fuchsin) is applied to a heat-fixed smear.

• Purpose: To determine the shape, size, and arrangement of bacteria.
• Types: Direct (positive) staining — the cell takes up the dye; negative staining — the background is stained (e.g., using India ink) and the cell appears as a clear halo (used to demonstrate capsules).

2. Differential Staining

Uses more than one dye to differentiate bacteria into groups based on their properties.

(a) Gram Staining — the most widely used method.

• Steps: Primary stain (crystal violet) → mordant (Gram's iodine) → decolouriser (95% alcohol/acetone) → counterstain (safranin).
• Result: Gram-positive bacteria retain crystal violet and appear purple/violet (thick peptidoglycan layer); Gram-negative bacteria lose the dye and take up safranin, appearing pink/red (thin peptidoglycan, outer membrane).

(b) Acid-Fast Staining (Ziehl–Neelsen method)

• Steps: Primary stain (carbol fuchsin, with heat) → decolouriser (acid-alcohol) → counterstain (methylene blue).
• Result: Acid-fast bacteria (e.g., Mycobacterium tuberculosis) resist decolourisation and appear red; non-acid-fast bacteria appear blue.

3. Special Staining

Used to demonstrate specific bacterial structures.

4. Negative Staining

The dye stains the background while the bacteria remain unstained (colourless), useful for observing capsules and organisms that are difficult to stain.

5. Impregnation (Silver) Staining

Silver salts are deposited on the structure (e.g., flagella, spirochaetes), making them appear dark brown/black against a light background.

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Conclusion: Simple staining gives basic morphological information, differential staining (Gram and acid-fast) separates bacteria into groups, and special staining reveals specific cellular structures — together these methods form the foundation of bacterial identification.

Techniques for Isolation and Enumeration of Yeast

1. Isolation Techniques

a) Streak Plate Method A loopful of diluted sample is streaked over the surface of a solidified medium (e.g., Sabouraud Dextrose Agar or YM agar) in a pattern that progressively dilutes the cells. After incubation at 25–28 °C for 2–3 days, well-separated colonies develop, each arising from a single cell. Individual colonies are then picked and purified.

b) Pour Plate Method Serial dilutions of the sample are mixed with molten agar (cooled to ~45 °C) and poured into sterile Petri dishes. Yeast cells are trapped at different depths and develop into distinct colonies — both surface and subsurface.

c) Spread Plate Method A measured volume (0.1 mL) of diluted sample is spread evenly over the surface of a pre-dried agar plate using a sterile L-shaped glass spreader. This gives well-isolated surface colonies.

d) Enrichment and Selective Isolation

• Acidification: Media acidified to pH 3.5–4.5 with tartaric or citric acid suppresses bacterial growth and favours yeast.
• Antibiotics: Chloramphenicol, streptomycin or gentamicin are added to inhibit bacteria.
• Selective media: Media containing dyes or bile salts (e.g., Rose Bengal Agar, Biggy Agar) selectively isolate and differentiate yeast.
• Enrichment broth: Liquid medium (e.g., malt extract broth) is used to increase yeast numbers before plating.

e) Membrane Filtration For samples with low yeast counts (e.g., water, beverages), a known volume is passed through a membrane filter (0.45 µm pore). The filter is placed on agar and incubated; colonies grow on the membrane surface.

2. Enumeration Techniques

a) Standard Plate Count (Viable Count) Serial dilutions are plated (pour or spread plate) and incubated. Plates with 30–300 colonies are counted. Results are expressed as CFU/mL or CFU/g:

$\text{CFU/mL} = \frac{\text{Number of colonies}}{\text{Volume plated (mL)} \times \text{Dilution factor}}$

b) Most Probable Number (MPN) Method Serial dilutions are inoculated into replicate tubes of liquid medium (e.g., 3 or 5 tubes per dilution). Tubes showing growth (turbidity/gas) are recorded, and the MPN is read from standard MPN tables. Useful for samples with very low yeast density.

c) Direct Microscopic Count A known volume of sample is placed in a haemocytometer (Neubauer chamber) and cells are counted under the microscope. It is rapid but counts both live and dead cells.

$\text{Cells/mL} = \frac{\text{Number of cells counted} \times \text{Dilution}}{\text{Volume of chamber counted}}$

d) Membrane Filter Count After filtration and incubation, colonies on the membrane are counted and expressed per volume of sample filtered.

e) Turbidimetry / Viable Biomass Estimation Optical density (OD) at 600 nm or dry weight of cells is measured and correlated with cell number using a standard curve.

Summary Table

Key media used: Sabouraud Dextrose Agar (SDA), YM agar, Malt Extract Agar, Rose Bengal Agar (with chloramphenicol).

Intestinal protozoan parasites are unicellular eukaryotes living in the human gut. Their morphology is described in two stages: the trophozoite (motile, feeding, pathogenic) and the cyst (non-motile, resistant, infective).

General morphological characteristics

1. Unicellular and microscopic, usually 5–100 $\mu$m.
2. Definite, species-specific shape (pear-shaped, oval, round or irregular).
3. Cytoplasm differentiated into clear ectoplasm and granular endoplasm.
4. Locomotory organelles: pseudopodia (amoebae), flagella (flagellates) or cilia (ciliates).
5. Nucleus with a central karyosome and peripheral chromatin; ciliates have a macronucleus and micronucleus.
6. A cytostome (mouth) and cytopyge are present in ciliates.
7. Contractile and food vacuoles for osmoregulation and digestion.
8. Cyst has a thick protective wall, 1–8 nuclei (depending on species) and sometimes chromatoid bars.
9. Mostly anaerobic or microaerophilic; mitochondria are absent in Entamoeba and Giardia.

Examples

Other intestinal protozoa include Cryptosporidium parvum, Isospora belli and Cyclospora cayetanensis (coccidia, which form oocysts).

Contributions of Louis Pasteur and Robert Koch in Microbiology

Louis Pasteur (1822–1895) — Father of Microbiology

1. Disproved spontaneous generation (1859–1861): Through his swan-neck flask experiments, he showed that microorganisms arise from pre-existing microbes in the air, not from non-living matter. This finally settled the long-standing theory of abiogenesis.
2. Germ theory of disease: He established that specific microorganisms cause fermentation and spoilage, and extended this idea to disease, proving that microbes are the causal agents of infectious diseases.
3. Pasteurization: He developed the process of heating liquids (wine, beer, milk) to about 60–63 °C to kill spoilage microbes without altering taste — still used commercially today.
4. Vaccines: He developed vaccines against chicken cholera, anthrax (1881) and rabies (1885) using attenuated (weakened) pathogens. The rabies vaccine was first used successfully on Joseph Meister.
5. Other work: Discovered anaerobic life and named the process "fermentation"; introduced the terms "aerobes" and "anaerobes"; developed techniques of sterilization and pure culture that laid the foundation of modern microbiology.

Robert Koch (1843–1910) — Founder of Medical Bacteriology

1. Identified specific pathogens: He isolated and identified Bacillus anthracis (anthrax, 1876), Mycobacterium tuberculosis (tuberculosis, 1882) and Vibrio cholerae (cholera, 1883).
2. Koch's postulates (1884): He formulated four criteria to prove that a particular microorganism causes a particular disease — the microorganism must be present in all diseased hosts, be isolated and grown in pure culture, reproduce the disease when inoculated into a healthy host, and be re-isolated from the experimentally infected host. These postulates remain the gold standard of medical microbiology.
3. Pure culture techniques: He introduced solid media (potato slices, then gelatin and agar) for growing bacteria as isolated colonies, and developed the Petri dish (with Petri) and streak-plate method.
4. Staining techniques: He developed methods of staining bacteria (e.g., methylene blue) and discovered the tubercle bacillus using staining; he also described bacterial spores.
5. Recognition: Awarded the Nobel Prize in Physiology or Medicine in 1905 for his work on tuberculosis.

Conclusion: Pasteur established that microbes are the cause of fermentation and disease and gave the world pasteurization and vaccines, while Koch proved the germ theory experimentally through Koch's postulates and identified the causative agents of major killer diseases. Together they transformed microbiology from a descriptive science into an experimental and medical science.

Beneficial role of normal flora in human body

Normal flora (indigenous microbiota) are the microorganisms that permanently reside on the skin and mucous membranes of a healthy human body without causing disease. They play several beneficial roles:

1. Protection against pathogens (microbial antagonism): Normal flora occupy attachment sites and consume available nutrients, so invading pathogens cannot colonize. They also produce inhibitory substances such as fatty acids (skin), lactic acid (vagina), and bacteriocins that suppress harmful microbes. This is called colonization resistance.

2. Production of vitamins and nutrients: Gut bacteria such as Escherichia coli and other colonic flora synthesize vitamin K and several B-complex vitamins (biotin, folic acid, riboflavin, pyridoxine), which are absorbed and used by the host.

3. Aid in digestion and metabolism: They ferment undigested carbohydrates, break down complex polysaccharides, and help in the absorption of nutrients. They also assist in the metabolism of bile acids and drugs.

4. Development and maturation of the immune system: Exposure to normal flora stimulates the development of lymphoid tissues (e.g., Peyer's patches) and keeps the immune system "trained" and balanced. Germ-free animals show poorly developed immune organs.

5. Stimulation of antibody production: Normal flora induce the production of natural antibodies (cross-reacting antibodies) that provide cross-protection against related pathogens.

6. Anticancer and detoxifying effect: Intestinal flora help in the degradation of toxic compounds and carcinogens, reducing the risk of colon cancer.

7. Maintenance of physiological conditions: Vaginal lactobacilli keep the pH acidic (about pH 4–4.5) by producing lactic acid, preventing growth of Candida and other pathogens. Skin flora maintain the acidic pH of the skin.

Thus, normal flora live in a mutually beneficial (commensal or mutualistic) relationship with the host and are essential for health, digestion, immunity, and protection against infection.

(a) RNA: Ribonucleic acid is a single-stranded nucleic acid made of ribonucleotides (ribose sugar, phosphate and the bases A, U, G, C); it takes part in protein synthesis as mRNA, tRNA and rRNA.

(b) Neutralization test: An antigen–antibody test in which a specific antibody neutralizes the infectivity or toxicity of a virus or toxin, so that it no longer produces its effect in cell culture or in animals (e.g. viral neutralization test, antitoxin neutralization).

(c) Metabolism: The sum of all chemical reactions in a living cell, consisting of anabolism (building up, uses energy) and catabolism (breaking down, releases energy).

(d) Photolithotrophic bacteria: Bacteria that use light as the energy source and inorganic compounds (such as $H_2S$, S or $H_2$) as electron donors, e.g. green sulphur bacteria and purple sulphur bacteria.

(e) Methods of culture of anaerobic bacteria:

1. Anaerobic jar (McIntosh–Fildes jar) with a gas pak or catalyst
2. Robertson's cooked meat medium
3. Thioglycollate broth
4. Pyrogallol method (alkaline pyrogallol absorbs oxygen)
5. Anaerobic chamber or glove box
6. Roll-tube or deep agar culture (e.g. Wright's tube)

(f) Plasmid: A small, circular, extrachromosomal, self-replicating double-stranded DNA molecule in bacteria that often carries genes for antibiotic resistance or toxin production and is used as a cloning vector.

(g) Uses of dark field microscope: To observe live, unstained, very thin organisms that are invisible in a bright field, especially spirochaetes such as Treponema pallidum (syphilis) and Leptospira, and to study their motility.

(h) Bacterial capsule: A well-defined, gelatinous layer (usually polysaccharide, sometimes polypeptide) outside the cell wall that protects against phagocytosis and drying. Examples: Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Bacillus anthracis.

(i) Characteristics of nematodes:

1. Bilaterally symmetrical, triploblastic, unsegmented, cylindrical worms tapering at both ends, covered by a tough cuticle.
2. Pseudocoelom (false body cavity) present instead of a true coelom.
3. Complete digestive tract from mouth to anus.
4. Excretory system of renette cells or canals; circulatory and respiratory systems absent.
5. Sexes usually separate (dioecious) with sexual dimorphism.
6. Free-living or parasitic, e.g. Ascaris lumbricoides, Wuchereria bancrofti, Enterobius vermicularis.

(j) Glycogenolysis: The breakdown of stored glycogen into glucose-1-phosphate (then glucose-6-phosphate and glucose) by glycogen phosphorylase, mainly in liver and muscle, to supply glucose when blood sugar is low.

Mutation is a sudden, heritable change in the nucleotide sequence of DNA (or in the genetic material) of an organism. It may alter a single base pair or large segments of a chromosome, and it is transmitted to daughter cells or offspring. Mutations arise spontaneously (errors in replication, tautomeric shifts, depurination) or are induced by mutagens (UV light, X-rays, base analogues, alkylating agents). In bacteria, mutation is the primary source of genetic variation and the raw material for evolution and antibiotic resistance.

Types of Bacterial Mutation

Bacterial mutations are classified in several ways.

A. On the basis of extent of change (point vs. chromosomal)

1. Point mutation (gene mutation) — change in a single base pair.

• Substitution
  • Transition: purine → purine (A↔G) or pyrimidine → pyrimidine (C↔T).
  • Transversion: purine ↔ pyrimidine (A↔C, A↔T, G↔C, G↔T).
  • Silent (synonymous): new codon codes for the same amino acid; no change in protein.
  • Missense: new codon codes for a different amino acid (e.g. sickle-cell type change).
  • Nonsense: codon changed to a stop codon (UAA, UAG, UGA); truncated protein.
• Frameshift mutation: insertion or deletion of bases not in multiples of three, shifting the reading frame; usually produces a completely non-functional protein.
  • Insertion — addition of one or more bases.
  • Deletion — loss of one or more bases.

2. Chromosomal mutation (gross mutation) — changes in large DNA segments.

• Deletion — loss of a segment of the chromosome.
• Duplication — a segment is repeated.
• Inversion — a segment is reversed end to end.
• Translocation — a segment is transferred to another position or another chromosome.

B. On the basis of effect on phenotype (function)

• Silent mutation: no observable phenotypic change.
• Missense mutation: altered but partially functional protein.
• Nonsense mutation: premature stop, protein usually inactive.
• Frameshift mutation: reading frame disturbed, protein grossly altered.
• Null (loss-of-function) mutation: gene product completely lost.
• Lethal mutation: kills the cell or organism.
• Conditional mutation: expressed only under certain conditions (e.g. temperature-sensitive mutants grow at 30 °C but not at 42 °C).
• Suppressor mutation: a second mutation that restores the function lost by the first.
• Reversion (back mutation): restores the original wild-type phenotype.

C. On the basis of direction of change

• Forward mutation: wild type → mutant.
• Reverse (back) mutation: mutant → wild type (true reversion or suppression).

D. On the basis of cause (origin)

• Spontaneous mutation: occurs naturally without any external agent, e.g. replication errors, deamination of cytosine, tautomeric shifts.
• Induced mutation: produced by physical or chemical mutagens, e.g. UV light (pyrimidine dimers), X-rays (strand breaks), 5-bromouracil (base analogue), nitrous acid (deamination), acridine dyes (frameshift).

E. Special mutational changes in bacteria

• Auxotrophic mutation: loss of ability to synthesize an essential metabolite, so the mutant requires it in the medium (e.g. E. coli his⁻).
• Prototrophic reversion: auxotroph regains the ability to synthesize the metabolite.
• Antibiotic-resistance mutation: mutation in genes such as rpsL or gyrA confers resistance to streptomycin or quinolones.
• Phase variation: reversible switching of surface antigens (e.g. flagellar H antigens in Salmonella).
• Smooth ↔ rough (S–R) variation: mutation affecting the O-antigen/lipopolysaccharide, changing colony morphology from smooth to rough.

Molecular basis of bacterial mutation

Mutations arise by:

1. Replication errors — mispairing of bases due to tautomerization, slippage causing frameshifts.
2. Chemical damage — deamination, alkylation, oxidation, base loss (depurination/apyrimidinic sites).
3. Mutagens — base analogues incorporated in place of normal bases; intercalating agents causing frameshifts; UV causing thymine dimers.
4. Repair failure — errors persist when mismatch repair, photoreactivation, excision repair or SOS repair is defective (e.g. mut mutants).

Significance

Bacterial mutations produce genetic diversity, allow adaptation to new environments, cause antibiotic resistance (a major clinical problem), and are widely used in research to study gene function (auxotrophs, mutants in metabolic pathways).

Factors Affecting Bacterial Growth

Bacterial growth is the increase in the number of bacterial cells, not in cell size. It depends on a number of extrinsic (environmental) and intrinsic (nutritional and genetic) factors. The important ones are described below.

1. Temperature

Temperature is one of the most important factors. Every bacterium has a minimum, optimum and maximum growth temperature. Based on this, bacteria are grouped as:

• Psychrophiles — grow best at 0–15 °C (e.g., Pseudomonas, Flavobacterium).
• Mesophiles — grow best at 20–45 °C (e.g., E. coli, most pathogens).
• Thermophiles — grow best at 45–80 °C (e.g., Bacillus stearothermophilus).
• Hyperthermophiles — grow above 80 °C (e.g., Pyrococcus).

Low temperature slows metabolism (bacteriostatic), while high temperature denatures proteins and enzymes (bactericidal).

2. pH

Most bacteria grow best at neutral pH (6.5–7.5). Acidophiles grow at low pH (e.g., Lactobacillus, pH 3–4), while alkaliphiles grow at high pH (e.g., Vibrio cholerae, pH 8–9). Extreme pH denatures enzymes and inhibits growth. Buffers in media help maintain optimum pH.

3. Oxygen (Gaseous Requirement)

On the basis of oxygen requirement, bacteria are classified as:

• Obligate aerobes — require oxygen (e.g., Mycobacterium tuberculosis).
• Obligate anaerobes — killed by oxygen (e.g., Clostridium).
• Facultative anaerobes — grow with or without oxygen (e.g., E. coli).
• Microaerophiles — need low oxygen (e.g., Campylobacter).
• Aerotolerant anaerobes — tolerate oxygen but do not use it (e.g., Streptococcus).

Oxygen affects growth because it is used as a terminal electron acceptor in aerobic respiration and toxic oxygen radicals (superoxide, H₂O₂) damage anaerobic cells.

4. Moisture and Water Activity

Water is essential as a solvent for nutrients and for metabolic reactions. Bacteria require high water activity (a_w ≈ 0.90–0.99). Drying (desiccation) stops growth but does not always kill bacteria. Addition of salt or sugar lowers water activity and preserves food (e.g., salted fish, jam).

5. Nutrients

Bacteria need a source of carbon, nitrogen, phosphorus, sulphur, trace elements and growth factors (vitamins). Absence of any essential nutrient limits growth. For example, iron is required for cytochrome and is often a limiting factor in the host body.

6. Osmotic Pressure and Salt Concentration

Sudden changes in osmotic pressure cause plasmolysis (in hypertonic solution) or plasmotysis (in hypotonic solution). Halophiles tolerate high salt; osmophiles tolerate high sugar. Most bacteria are inhibited in high salt or sugar concentrations.

7. Carbon Dioxide

Some bacteria (capnophiles) require 5–10 % CO₂ for growth (e.g., Neisseria, Brucella). CO₂ is used in carboxylation reactions and for capsule formation.

8. Light and Radiation

Ultraviolet light and ionizing radiation are mutagenic and lethal. Visible light may be harmful in the presence of photosensitizing dyes. Most bacteria grow best in the dark.

9. Hydrostatic Pressure

Barophilic (piezophilic) bacteria grow at high hydrostatic pressure in deep sea. High pressure affects enzyme activity and membrane fluidity.

10. Other Factors

• Redox potential (Eh) — anaerobes need low Eh; aerobes need high Eh.
• Antibiotics and antimicrobial agents — inhibit cell wall, protein or nucleic acid synthesis.
• Bacteriocins — kill related bacterial strains.
• Host factors — in vivo growth is affected by body temperature, pH, immune response and normal flora.

Conclusion

Bacterial growth is thus the net result of several interacting factors. The optimum combination of temperature, pH, oxygen, moisture, nutrients and absence of inhibitory substances gives maximum growth. Any deviation from the optimum slows or stops growth, and this principle is widely used in food preservation, sterilization and chemotherapy.

Dry Heat Sterilization — Principle and Methods

Principle

Dry heat sterilization kills microorganisms by oxidation of cell constituents, protein denaturation, and burning (incineration) rather than by moist heat coagulation. Because there is no water or steam present, the killing is less efficient than moist heat at the same temperature — the moisture in steam greatly speeds up protein denaturation. Therefore dry heat requires higher temperatures and longer exposure times than autoclaving (e.g. 160 °C for 2 hours in a hot-air oven, compared with 121 °C for 15 minutes in an autoclave).

The mechanism is essentially:

• Oxidation of cellular proteins, lipids and nucleic acids
• Denaturation and dehydration of proteins
• Incineration (complete burning to ash) at very high temperatures

Dry heat is suitable for glassware, metal instruments, oils, powders, and other moisture-sensitive or heat-stable materials that would be damaged or corroded by steam.

Methods of Dry Heat Sterilization

1. Red Heat (Red-hot) The article is held directly in a flame until it glows red. Used for platinum loops, wires, needles, forceps, and the mouths of test tubes/flasks. It is the simplest and most certain method, but applicable only to small metal objects that will not be damaged by burning.

2. Flaming The article is passed through a flame without being allowed to become red hot. Used for glass slides, cover slips, scalpels, and the mouths of culture tubes. Alcohol may be poured over the article and ignited (e.g. glass spreaders). Less reliable than red heat because the temperature attained is lower.

3. Incineration (Burning to ash) The material is completely burned to ashes in an incinerator or furnace at very high temperature. Used for disposable items — soiled dressings, bandages, animal carcasses, infected plastic/paper waste, and pathological waste. This is the method of choice for disposal of infectious waste.

4. Hot Air Oven (Hot-air oven / Poupinel's oven) The most widely used and controlled method. The oven is electrically heated and fitted with a fan (forced draught) to circulate hot air so that the temperature is uniform throughout. The articles are wrapped in paper or placed in metal containers and kept in the oven.

Standard conditions:

• 160 °C for 2 hours, or
• 170 °C for 1 hour, or
• 180 °C for 30 minutes

Used for glassware (test tubes, petri dishes, pipettes, flasks), metal instruments (scissors, forceps, scalpels), glass syringes, oils, fats, waxes, and powders (talc, sulphonamides). It is not suitable for rubber, plastic, fabrics, or any heat-labile material. The oven must not be overloaded, as air must circulate freely; wrapping should be loose. A Browne's tube (containing a heat-sensitive red dye that turns green) or thermocouple is used to monitor that the required temperature has actually been reached inside the load.

5. Infrared Radiation Infrared rays are used to sterilize surfaces of instruments and glassware in specially designed chambers. The heat is absorbed by the surface and converted to thermal energy. Used mainly in industry and for rapid surface sterilization of small instruments.

6. Glass Bead Sterilizer Small metal instruments (e.g. scissors, forceps, burrs) are thrust into a bed of glass beads heated to about 250 °C for a few seconds. Provides rapid chair-side sterilization in dental and minor surgical practice.

7. Flaming by Burning Alcohol / Burning Method Articles are dipped in or moistened with alcohol and ignited; the burning alcohol provides the sterilizing heat. Used for glass rods, spreaders, and small trays.

Advantages and Limitations

Conclusion

Dry heat sterilization works by oxidation and incineration of microbial cells and is therefore applied where moisture is undesirable. The hot-air oven (160 °C/2 h) is the standard method for glassware and metal instruments, while red heat, flaming, incineration, infrared radiation and glass-bead sterilizers cover special situations such as loops, slides, and infectious waste.

General structure of virus

A virus is a submicroscopic, obligate intracellular parasite consisting of a nucleic acid core surrounded by a protein coat. It is not a cell and has no independent metabolism. A complete, infectious virus particle is called a virion. Its general structure has the following parts:

1. Nucleic acid (core/genome): Either DNA or RNA, never both. The nucleic acid may be single-stranded (ss) or double-stranded (ds), linear or circular, and segmented or non-segmented. It carries the genetic information for replication.

2. Capsid: The protein coat made of repeating subunits called capsomeres, which are themselves made of polypeptide chains (protomers). The capsid protects the nucleic acid and helps in attachment to the host cell. Its arrangement gives three basic shapes:

   • Helical (e.g., Tobacco Mosaic Virus) — capsomeres arranged in a spiral around the nucleic acid.
   • Icosahedral/cubical (e.g., Adenovirus) — 20 triangular faces, 12 vertices.
   • Complex (e.g., Bacteriophage T4) — head (icosahedral) plus helical tail with fibres.

3. Nucleocapsid: The combined nucleic acid + capsid unit.

4. Envelope: An outer lipoprotein membrane present only in enveloped viruses (e.g., Influenza, HIV). It is derived from host cell membranes and carries virus-specific spikes/peplomers (glycoproteins) used for attachment (e.g., haemagglutinin, neuraminidase). Enveloped viruses are generally more fragile than naked ones.

5. Matrix protein (in some enveloped viruses): Lies between the nucleocapsid and envelope, giving stability.

6. Enzymes (in some viruses): e.g., reverse transcriptase in retroviruses, lysozyme in bacteriophages.

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Classification schemes of viruses

Viruses are classified on the basis of several criteria:

A. Based on host range

1. Bacteriophages (bacterial viruses): infect bacteria, e.g., T4 phage, λ phage.
2. Phytophages/plant viruses: infect plants, e.g., TMV, Potato virus X.
3. Zoophages/animal viruses: infect animals and humans, e.g., Poliovirus, Influenza virus.

B. Based on nucleic acid type

1. DNA viruses: e.g., Adenovirus (ds DNA), Parvovirus (ss DNA).
2. RNA viruses: e.g., Influenza virus (ss RNA), Reovirus (ds RNA).
3. DNA–RNA intermediate (retroviruses): RNA genome that makes DNA copy, e.g., HIV.

C. Based on structure (naked vs enveloped)

1. Naked (non-enveloped) viruses: only nucleocapsid, e.g., Poliovirus, Adenovirus.
2. Enveloped viruses: nucleocapsid surrounded by lipoprotein envelope, e.g., HIV, Herpes simplex virus.

D. Based on capsid symmetry (morphology)

1. Helical: TMV, Influenza virus (helical nucleocapsid).
2. Icosahedral (cubical): Adenovirus, Poliovirus.
3. Complex: Poxviruses, Bacteriophage T4.
4. Binary symmetry: T-even phages (head + tail).

E. Based on genome structure

1. Linear or circular nucleic acid.
2. Segmented or non-segmented genome, e.g., Influenza virus has 8 segmented RNA molecules.

F. Based on replication site

1. Nuclear replicating: Adenovirus, Herpes virus.
2. Cytoplasmic replicating: Poxvirus, most RNA viruses.

G. Based on the number of strands

1. Single-stranded (ss): positive-sense (+) or negative-sense (−) RNA.
2. Double-stranded (ds): ds DNA or ds RNA.

H. Baltimore classification (modern scheme) This is the most widely accepted scheme, based on the relationship between the viral genome and the mRNA needed for protein synthesis. It divides viruses into seven groups:

I. ICTV classification The International Committee on Taxonomy of Viruses classifies viruses hierarchically into Order → Family → Subfamily → Genus → Species, using genome type, morphology, and serological relationships. Virus names are written in italics (e.g., Human immunodeficiency virus).

Thus, viruses are grouped by host, nucleic acid, morphology, envelope, and replication strategy, with the Baltimore system being the most comprehensive modern scheme.

Detection Techniques of Helminthic Parasites

Helminths (parasitic worms) are detected in the laboratory mainly by microscopic examination of stool, supported by blood tests, serology, and molecular methods. The choice of technique depends on the parasite and its stage (egg, larva, adult worm, or cyst) present in the specimen.

1. Direct Microscopic Examination of Stool

a) Saline wet mount A small amount of stool is emulsified in normal saline on a slide and examined under low and high power. Saline reveals motile trophozoites and live larvae and shows the morphology of eggs (e.g., Ascaris, Trichuris, hookworm, Taenia, Hymenolepis). Iodine mount is used in parallel to stain nuclei and glycogen for better contrast.

b) Concentration techniques Used when the parasite load is low. They separate eggs/larvae from fecal debris by density differences.

• Sedimentation (formol–ether concentration): Stool is sieved, washed, and centrifuged with formalin and ether. The sediment contains eggs, larvae, and cysts. It is the method of choice for most helminth eggs and is also good for schistosome eggs.
• Flotation (saturated salt or zinc sulphate solution): Eggs lighter than the flotation fluid rise to the surface and are collected on a coverslip. Useful for Ascaris, hookworm, and Trichuris eggs, but not for heavy or operculated eggs (e.g., unfertilized Ascaris eggs, Fasciola eggs) or schistosome eggs.
• Kato–Katz thick smear: A measured amount of stool is pressed through a template and cleared with glycerol–malathion. It is quantitative (eggs per gram) and is the standard field method for schistosomiasis and soil-transmitted helminths.

2. Stool Examination for Larvae and Adult Worms

• Baermann funnel technique: Fresh stool is placed on gauze in a funnel of warm water; motile larvae migrate downward and are collected in the stem. Used for hookworm and Strongyloides stercoralis larvae.
• Harada–Mori filter paper culture: Stool on filter paper is cultured for 7–10 days; larvae migrate to the water at the bottom. It differentiates hookworm and Strongyloides and is useful when egg output is low.
• Macroscopic examination: Whole stool is examined for adult worms (e.g., Ascaris, Taenia proglottids, Enterobius) and for proglottids of tapeworms after clearance.

3. Anal Swab / Perianal Examination

• Graham's Scotch tape method: A clear adhesive tape is pressed against the perianal skin in the morning before bathing and examined microscopically. It is the method of choice for Enterobius vermicularis eggs and also detects Taenia eggs.

4. Blood and Tissue Examination

• Blood smear: Thick and thin Giemsa-stained smears detect microfilariae of Wuchereria bancrofti, Brugia, Loa loa, and Onchocerca (skin snip for the latter).
• Knott's concentration: Blood is lysed with formalin and centrifuged; the sediment is examined for microfilariae, useful in low-density infections.
• Tissue biopsy / aspirate: Muscle biopsy for Trichinella larvae; rectal or bladder biopsy for schistosome eggs; aspirate of hydatid cyst fluid for Echinococcus hooklets and protoscolices.
• Urine examination: Midday urine sedimentation detects Schistosoma haematobium eggs; also used for Trichomonas (not helminth) and occasionally for Enterobius.

5. Immunological and Serological Tests

Used when parasites are not found in stool or blood, or for tissue-dwelling helminths.

• ELISA / indirect haemagglutination: Detects antibodies or antigens in cysticercosis, hydatid disease, trichinosis, strongyloidiasis, filariasis, and schistosomiasis.
• Rapid antigen tests: Circulating antigen detection for Wuchereria bancrofti (filariasis) and coproantigen ELISA for Echinococcus and Taenia.
• Complement fixation and immunoblot: Confirmatory tests for some tissue helminths.

6. Molecular Techniques

• PCR and real-time PCR: Detect parasite DNA in stool, blood, or tissue; highly sensitive and specific for Strongyloides, Schistosoma, Echinococcus, and Taenia, and useful when microscopy is negative.
• DNA sequencing / genotyping: Helps in species identification and epidemiological studies.

7. Imaging and Other Adjuncts

• Ultrasound, CT, and MRI: Demonstrate hydatid cysts, neurocysticercosis, and hepatosplenic schistosomiasis.
• Endoscopy / colonoscopy: Visualize and biopsy adult worms in the gut (e.g., Ascaris, hookworm).
• Eosinophil count and IgE levels: Supportive evidence of helminthic infection.

Conclusion

No single technique detects all helminths. A rational approach combines direct and concentration stool microscopy for intestinal helminths, anal swab for Enterobius, blood/skin/tissue examination for filarial and tissue parasites, and serology, antigen detection, and PCR for low-load or tissue-dwelling infections. Correct specimen collection, timing (e.g., morning Scotch tape, midday urine), and repeated sampling greatly improve detection.

EMP Pathway (Glycolysis) and ATP Production from One Glucose Molecule

1. Introduction

The EMP pathway (Embden–Meyerhof–Parnas pathway), commonly called glycolysis, is the universal, oxygen-independent sequence of reactions by which one molecule of glucose is degraded to two molecules of pyruvate. It was worked out by Gustav Embden, Otto Meyerhof and Jacob Parnas (1920s).

• Site: cytoplasm (cytosol) of all living cells — prokaryotes and eukaryotes.
• Nature: anaerobic; needs no oxygen, but occurs in aerobic cells also as the first stage of respiration.
• Net result: 1 glucose → 2 pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O.
• Significance: only source of ATP in anaerobic organisms; supplies pyruvate for Krebs cycle, for lactic acid/ethanol fermentation, and carbon skeletons for biosynthesis.

The pathway has 10 enzymatic steps grouped into three phases.

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2. Description of the Pathway

Phase I — Preparatory (Investment) Phase

Glucose is phosphorylated and split; 2 ATP are consumed.

Phosphorylation at steps 1 and 3 "traps" the sugar inside the cell and activates it. Step 3 is the committed, rate-limiting step, regulated by ATP (inhibitor), AMP/ADP and fructose-2,6-bisphosphate (activators).

Phase II — Oxidation (Pay-off) Phase

Each of the two G3P molecules is oxidized and converted to pyruvate, generating 4 ATP and 2 NADH.

Key points

• Step 6 is the only oxidation–reduction step of glycolysis; NADH formed here is reoxidized by the electron transport system (aerobic) or by lactate/ethanol fermentation (anaerobic).
• Steps 7 and 10 are the two substrate-level phosphorylation steps; each occurs twice per glucose, giving 4 ATP.
• Only steps 1, 3 and 10 (hexokinase, PFK-1, pyruvate kinase) are irreversible under cell conditions; the other steps, including 6 and 7, are reversible.
• Overall equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{CH}_3\text{COCOOH} + 2\text{NADH} + 2\text{H}^+ + 2\text{ATP} + 2\text{H}_2\text{O}$

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3. Calculation of ATP Production from One Glucose Molecule

(a) Direct (substrate-level) ATP

$\text{Net ATP} = 4 - 2 = \mathbf{2\ ATP}$

(b) ATP from NADH (oxidative phosphorylation)

Two NADH are formed at step 6. In aerobic respiration they enter the mitochondria (via the glycerol-3-phosphate or malate–aspartate shuttle) and yield ATP through the ETS:

• Classical value: 1 NADH = 3 ATP → 2 NADH = 6 ATP
• Modern (current) value: 1 NADH = 2.5 ATP → 2 NADH = 5 ATP

(c) Total ATP yield

Answer: The EMP pathway alone gives a net gain of 2 ATP by substrate-level phosphorylation; when the 2 NADH are oxidized through the electron transport chain, the total yield becomes 8 ATP (classical value) or 7 ATP (modern value) per molecule of glucose. Under anaerobic conditions (fermentation), NADH is not oxidized by the ETS, so the net yield is only 2 ATP.

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4. Conclusion

The EMP pathway is the most primitive and universal route of glucose catabolism. It requires no oxygen, occurs in the cytoplasm, and converts one glucose into two pyruvate with a net gain of 2 ATP directly and up to 8 ATP in total when NADH is respired aerobically. It thus serves both as an energy-yielding mechanism and as the gateway to the Krebs cycle and fermentation.

Plasmid

A plasmid is a small, circular, extrachromosomal, double-stranded DNA molecule that replicates independently of the bacterial chromosome. It is not essential for the normal survival of the bacterium but often carries genes that give the host a selective advantage, such as antibiotic resistance, virulence factors, or metabolic capabilities. Plasmids are found mainly in bacteria (and also in some yeasts) and are transferred from one cell to another by conjugation, transduction, or transformation.

R-plasmid (Resistance plasmid)

An R-plasmid is a conjugative plasmid that carries genes conferring resistance to one or more antibiotics. It typically consists of two parts: the RTF (resistance transfer factor), which carries the genes for transfer (tra) functions, and one or more r-determinants, which carry the antibiotic-resistance genes. Many R-plasmids also carry multiple resistance genes, making them multiple-drug-resistance (MDR) plasmids.

Why R-plasmids are increasingly important in medical sectors

1. Spread of antibiotic resistance: R-plasmids are conjugative, so they can pass from resistant to sensitive bacteria within and between species (e.g., E. coli to Salmonella, Shigella, Klebsiella). This horizontal gene transfer rapidly spreads resistance genes through a bacterial population without the need for mutation or reproduction.

2. Multiple drug resistance (MDR): A single R-plasmid may carry resistance genes for several antibiotics at once (e.g., ampicillin, tetracycline, streptomycin, chloramphenicol, sulfonamides). Infection with such a strain leaves very few therapeutic options, increasing treatment failure and mortality.

3. Nosocomial (hospital-acquired) infections: Hospitals are ideal environments for R-plasmid transfer because of heavy antibiotic use and close contact between patients. MDR strains of Staphylococcus aureus (MRSA), Pseudomonas, Acinetobacter, and ESBL-producing Enterobacteriaceae are major causes of hospital outbreaks.

4. Failure of empirical therapy: Because resistance is unpredictable and spreads quickly, doctors cannot rely on previously effective first-line antibiotics. This forces the use of newer, costlier, and more toxic drugs (e.g., carbapenems, colistin), raising treatment cost and side effects.

5. Threat to public health and global spread: R-plasmids can move between humans, animals, and the environment (via food, water, and agricultural antibiotic use), so resistance can spread globally. The WHO has declared antimicrobial resistance one of the top ten global public health threats.

6. Complication of surgery, chemotherapy, and intensive care: These procedures depend on effective antibiotic cover. R-plasmid-mediated resistance makes them riskier and can lead to untreatable infections.

For these reasons, R-plasmids are of great medical importance: they are a major vehicle for the emergence and rapid dissemination of antibiotic resistance, and they directly threaten the success of modern medicine.

Principle of Phase Contrast Microscope

The phase contrast microscope, developed by Frits Zernike (1934, Nobel Prize 1953), is used to observe transparent, unstained living specimens (such as bacteria, protozoa, and tissue cells) that differ from their surroundings only in refractive index, not in colour or absorption.

Basic principle:

1. Phase difference instead of amplitude difference. Light passing through a transparent specimen is not absorbed, so no contrast in brightness (amplitude) is produced. However, because the specimen has a slightly different refractive index and thickness than the surrounding medium, the light passing through it is retarded (phase-shifted) relative to the light passing through the background. The amount of retardation is given by:

$\phi = \frac{2\pi}{\lambda}\,(\mu_s - \mu_m)\,t$

where $\mu_s$ = refractive index of specimen, $\mu_m$ = refractive index of medium, $t$ = thickness of specimen, $\lambda$ = wavelength of light. Typically this phase shift is about ¼ wavelength ($\lambda/4$).

2. Conversion of phase difference into amplitude (brightness) difference. The human eye cannot detect phase differences, only intensity differences. The microscope therefore converts the invisible phase shift into a visible light–dark contrast by interference of two sets of light rays:

   • Direct (undeviated) light — passes straight through the specimen and is brought to focus at the centre of the objective's back focal plane.
   • Diffracted (deviated) light — scattered by the specimen's internal structures and travels through the whole aperture of the objective.

3. Role of the phase plate. A phase plate (annular phase ring) is placed at the back focal plane of the objective, exactly where the direct light is focused. It does two things:

   • It advances or retards the direct light by a further $\lambda/4$ so that the total phase difference between direct and diffracted light becomes 0 or $\lambda/2$.
   • It reduces the amplitude of the direct light (by a partially absorbing film), so that the two beams become comparable in intensity and can interfere effectively.

4. Interference and image formation. The direct and diffracted rays are recombined at the image plane. If they are in phase, they interfere constructively (bright); if out of phase by $\lambda/2$, they interfere destructively (dark). Thus regions of higher refractive index (denser structures) appear dark against a lighter background in a positive phase contrast microscope (and vice versa in a negative one).

Result: The specimen appears with good contrast — dense parts dark, less dense parts lighter — even though it is unstained and transparent, allowing observation of living cells without fixation or staining.

Germ Theory of Disease

The germ theory states that many diseases are caused by the invasion and multiplication of microorganisms (germs) in the body. Its main points are:

1. Microorganisms cause disease — Specific microscopic organisms (bacteria, viruses, fungi, protozoa) are the causative agents of many infectious diseases.
2. Specificity of pathogens — Each infectious disease is caused by a specific type of microorganism (e.g., Mycobacterium tuberculosis causes tuberculosis).
3. Entry and multiplication — The pathogen must enter the host body, reach the susceptible tissue, and multiply there to produce disease.
4. Isolation and culture — The causative organism can be isolated from the diseased host and grown in pure culture outside the body.
5. Reproduction of disease — When the pure culture of the organism is introduced into a healthy susceptible host, it reproduces the same disease.
6. Prevention and control — Disease can be prevented and controlled by killing or blocking the pathogen through hygiene, antiseptics, disinfectants, antibiotics, and vaccines.

These points were established by the works of Louis Pasteur ( disproved spontaneous generation, showed microbes cause fermentation and disease) and Robert Koch (Koch's postulates), and were supported by Joseph Lister (antiseptic surgery) and Edward Jenner (vaccination).

Role of Microorganisms in the Food Sector

Microorganisms — bacteria, yeasts, moulds and viruses — play a central and double-edged role in the food sector. They are used deliberately in food production, they spoil food, and they cause food-borne disease. Their main roles are as follows:

1. Beneficial role in food production (fermentation)

• Dairy products: Lactobacillus species ferment lactose to lactic acid, curdling milk into curd, yoghurt, cheese and buttermilk. Streptococcus thermophilus and Lactobacillus bulgaricus are used in yoghurt.
• Bakery: Saccharomyces cerevisiae (baker's yeast) ferments sugars in dough, producing CO₂ (which raises the dough) and ethanol (which evaporates during baking).
• Alcoholic beverages: Yeasts convert sugars of grapes, barley and rice into ethanol for wine, beer and jand.
• Fermented foods: Moulds and bacteria are used in kinema, gundruk, sinki, marcha, idli, dosa, soy sauce and pickles.
• Organic acids and additives: Acetobacter produces vinegar (acetic acid); Aspergillus niger produces citric acid; Penicillium produces gluconic acid.

2. Food enrichment and value addition

• Microbial fermentation improves digestibility, flavour, texture and nutritive value (e.g., B-vitamins, essential amino acids) and removes anti-nutritional factors.
• Single Cell Protein (SCP): Spirulina, Chlorella and yeasts grown on cheap substrates give protein-rich biomass used as food/feed.
• Probiotics: Live Lactobacillus and Bifidobacterium in fermented milk products improve gut health.
• Enzymes: Microbial enzymes such as amylase, protease, pectinase, rennin (chymosin) and invertase are used in baking, cheese-making, juice clarification and meat tenderising.
• Vitamins and amino acids: Ashbya gossypii (riboflavin), Pseudomonas denitrificans (vitamin B₁₂) and Corynebacterium glutamicum (glutamic acid/MSG, lysine).

3. Food spoilage

• Saprophytic bacteria (Pseudomonas, Clostridium, Bacillus), yeasts and moulds (Aspergillus, Penicillium, Rhizopus) grow on food and cause souring, rotting, sliminess, off-flavour, discolouration and toxin production, making food unfit for consumption.

4. Food-borne disease and public health

• Pathogens such as Salmonella, Staphylococcus aureus, Clostridium botulinum, E. coli and Vibrio cholerae cause food poisoning and infections.
• Moulds like Aspergillus flavus produce aflatoxin, a carcinogenic mycotoxin.

5. Food preservation and safety

• Beneficial microbes produce acids, alcohol and bacteriocins that inhibit spoilage organisms and act as natural preservatives.
• Microorganisms are used in quality testing of food and water (MPN test, coliform test) and in producing antibiotics used in animal husbandry.

Conclusion: Microorganisms are indispensable in the food sector — as agents of fermentation, food enrichment and preservation on one hand, and as agents of spoilage and disease on the other. Controlled use of beneficial microbes and strict control of harmful ones are both essential for a safe and productive food industry.

MacConkey agar as a selective and differential medium

MacConkey agar contains bile salts and crystal violet as selective agents, and lactose with the pH indicator neutral red as the differential system.

Selective action

• Bile salts and crystal violet inhibit the growth of Gram-positive bacteria (e.g., Staphylococcus, Streptococcus) and many fastidious organisms.
• They allow the growth of Gram-negative bacteria, especially members of the Enterobacteriaceae (e.g., Escherichia coli, Salmonella, Shigella).
• Thus only a restricted group of organisms grows, which makes the medium selective.

Differential action

• Lactose is the fermentable carbohydrate, and neutral red is the indicator (colourless above pH 6.8, red below pH 6.8).
• Lactose fermenters (e.g., E. coli) produce acid from lactose fermentation, which lowers the pH and turns the colonies and surrounding medium pink to red (with bile salt precipitate around colonies).
• Non-lactose fermenters (e.g., Salmonella, Shigella, Pseudomonas) do not ferment lactose, so no acid is produced; they form colourless or pale colonies on the same plate.
• Therefore a single plate distinguishes lactose fermenters from non-lactose fermenters, which makes the medium differential.

Conclusion Because bile salts and crystal violet selectively permit only Gram-negative enteric bacteria to grow, and lactose with neutral red differentiates them by colony colour according to their lactose-fermenting ability, MacConkey agar is both a selective and a differential medium.

Functions of different types of RNA

RNA (ribonucleic acid) is a single-stranded polynucleotide that performs several vital roles in the cell. The three main types are:

1. Messenger RNA (mRNA)

• Carries the genetic message (codons) copied from DNA in the nucleus to the ribosomes in the cytoplasm.
• Acts as a template for protein synthesis during translation.
• Each set of three bases (codon) on mRNA specifies one amino acid.

2. Transfer RNA (tRNA)

• Transfers specific amino acids from the cytoplasm to the ribosome during protein synthesis.
• Has an anticodon loop that pairs with the complementary codon on mRNA, ensuring the correct amino acid is placed in the growing polypeptide chain.
• Also called soluble RNA (sRNA); it is the smallest of the three types.

3. Ribosomal RNA (rRNA)

• A structural and functional component of ribosomes (along with proteins).
• Provides the site for protein synthesis and helps in binding mRNA and tRNA.
• Catalyses peptide bond formation between amino acids (acts as a ribozyme).

Other types

• hnRNA (heterogeneous nuclear RNA): precursor of mRNA, processed into mature mRNA.
• snRNA (small nuclear RNA): involved in splicing of pre-mRNA.
• miRNA and siRNA: regulate gene expression by silencing or degrading specific mRNAs.

Conclusion: In short, mRNA carries the code, tRNA brings the amino acids, and rRNA provides the machinery — together they carry out the central process of protein synthesis.

MR-VP test

The MR-VP test is a composite biochemical test performed on a single glucose phosphate broth (MR-VP broth) culture to detect two different metabolic pathways by which bacteria ferment glucose. It consists of two separate tests — the Methyl Red (MR) test and the Voges-Proskauer (VP) test — read from the same culture after incubation.

Principle of the MR test

The MR test detects organisms that carry out mixed acid fermentation of glucose. Such bacteria (e.g. Escherichia coli) produce large amounts of stable organic acids — lactic, acetic, formic and succinic acids — which lower the pH of the medium to about 4.4 or below. The pH indicator methyl red is yellow at pH 6.2 and above but turns red at pH 4.4 and below. Hence, a red colour after adding methyl red indicates a positive MR test (acid production), while a yellow colour indicates a negative test.

Principle of the VP test

The VP test detects organisms that carry out the butylene glycol fermentation pathway. Such bacteria (e.g. Enterobacter aerogenes) convert glucose to the neutral end product 2,3-butanediol (acetoin is the intermediate), so the pH does not fall markedly. In the test, Barritt's reagent (α-naphthol and 40% KOH) is added; atmospheric oxygen oxidises acetoin to diacetyl, which then reacts with the guanidine group of peptone (creatine) in the broth to form a red-coloured complex. A cherry-red colour within 15–30 minutes is a positive VP test, while no colour change is negative.

Summary of the principle

Thus, the MR-VP test differentiates bacteria on the basis of the type of end products formed during glucose fermentation — acid producers give a positive MR test, whereas neutral acetoin producers give a positive VP test.

Bacterial Growth Curve

When bacteria are grown in a closed (batch) culture with a fixed amount of nutrient medium, the population does not increase at a constant rate. If the logarithm of the number of viable cells is plotted against time, a characteristic curve with four distinct phases is obtained. This is called the bacterial growth curve.

1. Lag phase When bacteria are first inoculated into a fresh medium, there is no immediate increase in cell number. The cells adapt to the new environment, synthesise enzymes, ribosomes, ATP and other cell components, and increase in size but not in number. The length of this phase depends on the age of the inoculum and the composition of the medium. It may be absent if the inoculum is taken from an actively growing culture into the same medium.

2. Log phase (exponential phase) The cells now divide at a constant, maximum rate, so the number increases geometrically (1, 2, 4, 8 …). Growth is balanced: every cell component is synthesised at the same relative rate. A straight line with the steepest slope is obtained on the logarithmic plot. The generation time is shortest and constant in this phase. This phase is used for biochemical and genetic studies because the cells are uniform and physiologically active. It ends when nutrients become limiting or toxic products accumulate.

3. Stationary phase The total number of cells remains constant because the rate of cell division equals the rate of cell death. This happens due to depletion of an essential nutrient, accumulation of toxic metabolites (acids, alcohols), and unfavourable changes in pH or oxygen supply. The cells become smaller, store reserve materials, and synthesise secondary metabolites (e.g. antibiotics, toxins, endospores). Growth is unbalanced.

4. Death (decline) phase The number of viable cells decreases exponentially as the death rate exceeds the division rate. Cells die due to continued nutrient exhaustion and accumulation of toxic products. Some cells may survive for long periods, and spore formers survive as endospores.

Significance The curve is important in food microbiology (spoilage), industrial fermentation (products are harvested at late log or stationary phase), antibiotic production, and in sterilisation and preservation studies.

Identification of Fungi in the Laboratory

Fungi are identified in the laboratory by studying their macroscopic (colony) characters and microscopic characters, supported by cultural and biochemical tests. The main steps are:

1. Macroscopic (Cultural) Examination

The fungus is grown on suitable media (e.g., Sabouraud's Dextrose Agar, Potato Dextrose Agar, Czapek-Dox Agar) and the colony is studied for:

• Rate of growth (rapid, moderate, slow)
• Colony texture (cottony, woolly, velvety, powdery, granular, yeast-like)
• Colony colour (surface and reverse)
• Topography (flat, raised, folded, crateriform)
• Pigment production and diffusible pigment in the medium

2. Microscopic Examination

A small portion of the colony is teased out or a slide culture/tape mount is prepared in lactophenol cotton blue (which kills the fungus and stains the wall) and observed under the microscope for:

• Septation of hyphae (septate or aseptate/coenocytic)
• Hyphal type (branching, spiral, racquet, nodular)
• Asexual spores/conidia — their shape, size, arrangement (e.g., conidia in chains, clusters, or singly), and the type of conidiophore (e.g., Aspergillus head, Penicillium brush, Fusarium sickle-shaped macroconidia)
• Sexual spores — ascospores, basidiospores, zygospores, where present
• Special structures — sporangia, sporangiophores, chlamydospores, arthrospores, clamp connections

3. Staining and Special Techniques

• Lactophenol cotton blue — routine mounting and staining
• Gram stain / KOH mount — for direct examination of clinical specimens (KOH clears host cells, revealing fungal elements)
• India ink — to demonstrate the capsule of Cryptococcus neoformans
• Slide culture technique — preserves the natural arrangement of spores and conidiophores
• Slide agglutination, ELISA, PCR — for rapid and specific identification of difficult or slow-growing fungi

4. Growth and Biochemical Tests

• Germ tube test — Candida albicans forms germ tubes in serum at 37 °C
• Sugar fermentation and assimilation tests — for identifying yeasts
• Urease test — positive in Cryptococcus
• Growth at different temperatures (25 °C, 37 °C, 45 °C) and cycloheximide tolerance help differentiate species

Conclusion: A fungus is identified by combining colony morphology on selective media, microscopic spore and hyphal features after staining, and specific biochemical or molecular tests — no single test is sufficient.

Non-specific immunity (also called innate or natural immunity) is the body's inborn, first-line defence system that protects against all foreign agents in a general way, without recognising or remembering any particular pathogen. It is present from birth and acts immediately or within hours of exposure. It has no memory and gives the same response on every encounter.

Components of non-specific immunity

1. Physical (anatomical) barriers

• Skin: intact epidermis is a physical barrier; sweat and sebum make the surface slightly acidic (pH 3–5) and inhibit microbes.
• Mucous membranes: line the respiratory, digestive and urogenital tracts; mucus traps microbes, which are then swept out by cilia (e.g. in the trachea).
• Hairs and cilia in the nose and airways filter inhaled particles.

2. Physiological (chemical) barriers

• Lysozyme in tears, saliva and nasal secretions dissolves bacterial cell walls.
• HCl of gastric juice kills swallowed microbes.
• Acidic pH of the vagina and fatty acids of sweat inhibit growth.
• Interferons are antiviral proteins secreted by virus-infected cells that make neighbouring cells resistant to viral attack.
• Complement proteins lyse microbes and enhance phagocytosis.
• Natural killer (NK) cells destroy virus-infected and tumour cells.

3. Phagocytic (cellular) barriers

• Neutrophils and monocytes/macrophages engulf and digest microbes by phagocytosis.
• Macrophages are found in blood, lymph, lungs (alveolar macrophages), liver (Kupffer cells) and spleen.

4. Inflammatory response

• On injury or infection, histamine from mast cells causes vasodilation and increased capillary permeability.
• This produces the four signs — redness, heat, swelling and pain — and brings more blood, phagocytes and antibodies to the site, localising and destroying the invader.

5. Fever

• Pyrogens raise body temperature, which speeds up phagocytosis and metabolic reactions and inhibits many pathogens.

Characteristics

• Present from birth; inherited; not acquired.
• Acts against a wide range of pathogens (non-specific).
• No memory, so no enhanced response on second exposure.
• Provides immediate but limited protection; it is the same in all normal individuals.

Significance: It prevents most infections from establishing, and it also initiates and supports the specific (acquired) immune response by presenting antigens to lymphocytes.

a. Nutritional types of bacteria

• Photoautotrophs: light as energy source, CO$_2$ as carbon source (cyanobacteria, purple sulphur bacteria).
• Photoheterotrophs: light as energy, organic compounds as carbon (purple non-sulphur bacteria).
• Chemoautotrophs (chemolithotrophs): energy from oxidising inorganic compounds, carbon from CO$_2$ (Nitrosomonas, Thiobacillus).
• Chemoheterotrophs (chemoorganotrophs): energy and carbon from organic compounds (E. coli, Bacillus, most pathogens).

b. Applications of SEM and TEM

• SEM: studies surface topography and three-dimensional shape of cells, colonies, biofilms, spores and flagella.
• TEM: studies internal ultrastructure (cell wall, membrane, ribosomes, nucleoid, plasmids), viruses and phage morphology, and thin sections of cells.

c. Thermophiles and psychrophiles

• Thermophiles: Thermus aquaticus, Bacillus stearothermophilus.
• Psychrophiles: Psychrobacter, Pseudomonas fluorescens (psychrotroph), Listeria monocytogenes (grows at refrigeration temperature).

d. AFB (acid-fast) staining

Ziehl–Neelsen staining detects acid-fast bacilli such as Mycobacterium tuberculosis and M. leprae, whose cell walls are rich in mycolic acid. The smear is stained with hot carbol fuchsin, decolourised with acid–alcohol (3% HCl in ethanol) and counterstained with methylene blue. Acid-fast bacteria resist decolourisation and appear red on a blue background; non-acid-fast bacteria appear blue. It is routinely used for sputum smears in TB diagnosis.

e. Oxidase-positive bacteria

Pseudomonas aeruginosa, Neisseria gonorrhoeae, Vibrio cholerae (also Helicobacter pylori, Campylobacter jejuni).

f. Features of the genetic code

Triplet; degenerate (more than one codon per amino acid); non-overlapping and comma-less; universal (with minor exceptions); unambiguous (one codon specifies one amino acid); has a start codon (AUG) and three stop codons (UAA, UAG, UGA); colinear with the polypeptide.

g. Chemoorganotrophic bacteria

Bacteria that obtain energy by oxidising organic compounds (e.g. glucose) and usually use organic compounds as the carbon source as well, for example E. coli, Staphylococcus aureus and Bacillus subtilis.

h. Haemagglutination test

Haemagglutination is the clumping of red blood cells by antibodies (against RBC antigens) or by viral/bacterial haemagglutinins (e.g. influenza virus). In the haemagglutination (HA) test, serial dilutions of a virus are mixed with a fixed RBC suspension; a diffuse mat of cells means agglutination (positive), while a button at the bottom means no agglutination. The highest dilution giving agglutination is the HA titre. In the haemagglutination inhibition (HI) test, specific antiserum blocks agglutination, which is used to identify viruses or measure antibody levels. Blood grouping is also based on this principle.

i. Three antibiotics

Penicillin, streptomycin, tetracycline (others: ampicillin, ciprofloxacin, erythromycin).

j. Differential media

MacConkey agar (lactose fermenters are pink, non-fermenters colourless), blood agar (haemolysis types), eosin methylene blue (EMB) agar (E. coli gives a green metallic sheen).